Like MATE galculator has a commitmet to GTK2 and GTK3 and is actively maintained.

Why ditch mate-calc?

While mate-calc is functional it lacks many features required of a
modern desktop calcualtor. The MATE team is small with limited resources
and where possible we are attempting to collaborate with projects that
add value to MATE and reduce development burden on the MATE team.
galculator is a perfect fit in this regard.

What’s in it for the galculator team?

galculator will remain an entirely independent project but we hope that its
inclusion in MATE will grow the galculator user base, raise awareness about
galculator and attract new contributors to their project. To that end the
MATE team will host the galculator translations as a resource on the
MATE transifex project.
We are hopeful that the ~300 heroic MATE translators will embrace galculator and
add to the ~20 languages galculator currently supports.

I am a packager. What does this mean for me?

If you are a MATE package maintainer then start creating galculator
packages for your distribution if they do not already exist and add
galculator to your MATE meta packages or package groups. MATE 1.10 is a
way off yet, so you have plenty of time.

Hopefully you undertand our reasoning and can see the mutual advantages
for this partnership. Let us know what you think in the comments.